This invention relates to focusing a laser and, in particular, to adjusting the focal point of a laser by detecting the location of a plasma generated by the laser.
Lasers are scanned for a variety of purposes including drilling holes in or otherwise machining a workpiece. For example, a laser may be used to machine glass or silicon wafers to form waveguides or microoptical structures. Laser machining is often performed by focusing a laser beam to a focal point. The focal point is a point of heightened power density which may advantageously be used for machining a workpiece. In order to use the focal point for precise machining applications, it may be necessary to precisely determine the location of the focal point so it may be accurately positioned on the workpiece.
Ultrafast lasers with pulse duration in the picosecond range and femtosecond range may be used for machining a workpiece. Ultrafast lasers may be used for precise ablation with little or no heat affected zone. They are especially suited for machining microscopic features in delicate materials such as thin films and brittle ceramics, which are otherwise difficult to machine. The focal point of ultrafast lasers must be accurately positioned for precise and accurate machining because of the small effective work area of ultrafast lasers.
The exact location of the focal point of a focused laser may be difficult to ascertain, however, due to deviations in the optical path. These deviations may be caused by abnormalities in mirrors used to direct the laser beam and in the focusing lens and by variations in the operating characteristics of the laser source. Thus, it is desirable to accurately adjust the position of the focal point with respect to the workpiece independent of variations in the laser source and the optical path.
The present invention provides a method and apparatus for focusing a laser relative to a workpiece using the position of a plasma formed by a laser. A laser beam is focused to a focal point to form a plasma. The intensity of the plasma formed by the focused laser beam is detected. The position of the focal point relative to the workpiece is adjusted responsive to the detected intensity of the plasma.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.